All total hydraulic failures on commercial aircraft have been caused by structural damage. A strengthened floor would have saved THY981 (along with McDD listening to Convair/General Dynamics' concerns about the floor, not getting the FAA to change their mind about the floor strength, fitting the supporting plate to prevent the torque tube from deforming, not falsifying records &c.) The addition of a fourth hydraulic system probably wouldn't have helped. Four hydraulic systems didn't save JAL123.
But a 747 DID have the same kind of accident like THY981. A cargo door opened, the floor duly failed and 9 passengers were blown into Pacific Ocean.
Unlike the DC-10 which fell out of the sky, the 747 kept flying and landed happily, minus the nine passengers sucked out. Something must have saved the 747.